AMR magnetic-field sensors are used in a plurality of applications and systems, for example in compasses, in systems for detecting ferromagnetic characteristics, in detecting electric currents, and in a wide range of other applications, by virtue of their capacity of detecting natural magnetic fields (for example, the Earth's magnetic field) and magnetic fields generated by electrical components (such as electrical or electronic devices and lines traversed by electric current).
As is known, such magnetoresistive sensors exploit the capacity of appropriate ferromagnetic materials (referred to as magnetoresistive materials, for example the material known as “permalloy” formed by a Fe—Ni alloy) of modifying their own resistance in the presence of an external magnetic field.
Currently, magnetoresistive sensors of an integrated type are manufactured as strips of magnetoresistive material on a substrate of semiconductor material, for example silicon. During manufacture, the magnetoresistive material strip is magnetized so as to have a preferential magnetization in a preset direction, for example the longitudinal direction of the strip.
In the absence of external magnetic fields, the magnetization maintains the imposed direction, and the strip has a maximum resistance. In the presence of external magnetic fields whose direction is different from the preferential magnetization, the angle between the magnetization of the strip and the current flowing therein changes, as its resistance, which decreases, as illustrated in FIGS. 1A and 1B.
In FIG. 1A, showing the magnetization M in the absence of an external magnetic field, a magnetoresistor 1 is formed by a magnetoresistive strip 2 having a longitudinal direction parallel to axis X (also referred to as the “easy axis”, since this direction is the direction of easier magnetization of the strip). The magnetoresistor 1 is supplied with a current I flowing in the longitudinal direction of the strip. In this condition, the magnetization M is parallel to the easy axis EA.
In FIG. 1B, the magnetoresistor 1 is immersed in an external magnetic field Hy parallel to axis Y (referred to as also as the “hard axis” since this is the direction of more difficult magnetization of the magnetoresistive strip 2). In this condition, the external magnetic field Hy causes a rotation of the magnetization M through an angle α with respect to the current I, causing a reduction of the resistance of the magnetoresistive strip 2 according to a law correlated to cos2α.
In order to linearize the plot of the resistance R at least in an operating portion of the curve, it is known to form, above the magnetoresistive strip 2, transverse strips 3 (referred to as “barber poles”), of conductive material (for example aluminum), arranged at a constant distance and with an inclination of 45° with respect to the direction of the easy axis EA, as illustrated in FIG. 2. In this situation, the direction of the current I changes, but not the magnetization M, and the resistance has a linear characteristic around the zero of the external magnetic field. In this way, it is possible to detect easily any magnetic fields oriented along or having a component parallel to the axis Y.
FIG. 3 shows a magnetoresistive sensor 4 including four magnetoresistors 1 of the type illustrated in FIG. 2, connected to form a Wheatstone bridge, with the transverse strips 3 arranged in an alternating way in each branch 4a and 4b of the bridge. The two branches 4a, 4b are connected at two input nodes 5, 6, with a biasing voltage Vb applied between them. In this way, the output voltage Vo between the intermediate nodes 7, 8 of the branches 4a, 4b is correlated to the existing external magnetic field Hy.
The magnetoresistive sensors of the type indicated above work properly provided that each magnetoresistor 1 is magnetized in the direction of the easy axis in the absence of external magnetic fields and as long as the imposed magnetization M persists.
In order to maintain the imposed magnetization M, magnetoresistive sensors generally comprise a set/reset coil (designated by 10 in FIG. 4). The set/reset coil 10 enables refresh operations, consisting in repeated rapid magnetization steps in the desired direction. As may be noted from FIGS. 4 and 5, the set/reset coil 10, here square spiral-shaped, has stretches 11 extending transversely, preferably perpendicular, to the longitudinal direction of the magnetoresistive strip 2 parallel to the easy axis EA. In the example illustrated (see in particular the cross-section of FIG. 5), the set/reset coil 10 is formed in a third metal level M3, above the magnetoresistive strip 2. In turn, the magnetoresistive strip 2 is formed below a first metal level M1, and the latter forms the barber poles 3. The described structure is further formed in an insulation structure 12 overlying a substrate 13, for example silicon, and forming with the latter an integrated chip 15.
In practice, during refresh, the set/reset coil 10 is supplied with a high current and generates a magnetic field B, which, in the area of the magnetoresistive strip 2, is parallel to the direction of the easy axis (see, for example, U.S. Pat. No. 5,247,278, incorporated by reference).
Currently available magnetoresistive sensors, which operate, for example, as linear or angular position sensors or as current sensors, may not, however, be easily used in industrial processes and in the automotive sector due to their rather reduced sensitivity scale.
There is a need in the art to provide a magnetoresistive sensor able to overcome the foregoing and other drawbacks.